Avoid Nearest mipmap filtering for BorderImage.

QSGOpaqueTextureMaterial has Nearest as the default mipmap
filtering, which BorderImage inherited. That default value is
not ideal, but I would prefer to not change behavior, so
request None from borderimage instead.

Change-Id: I9857f1434ae9f067c3b0e460ea552d3e854b9d64
Reviewed-by: aavit <eirik.aavitsland@digia.com>